arXiv subjects

Li Xiang

Publications and source records attributed to Li Xiang.

At least 37 records · Page 2Linked to original sources

arXiv2022

Magnetism and $T-x$ phase diagrams of Na and Ag substituted EuCd$_2$As$_2$

EuCd$_2$As$_2$ is an antiferromagnetic semimetal, that can host non-trivial topological properties, depending upon its magnetic state and excitations. Here, we report the synthesis and characterization of Eu(Cd$_{1-x}$Ag$_x$)$_2$As$_2$ and Eu$_{1-y}$Na$_y$Cd$_2$As$_2$, and study the evolution and nature of magnetic order with doping. Temperature-substitution phase diagrams are constructed from the electrical resistance and magnetic susceptibility data. We observe a splitting of the magnetic transition into two different transitions, and the gradual increase in one of the transition temperatures with Ag- and Na-substitution. The other transition remains more or less independent of doping. We further show that a magnetic state with a net ferromagnetic moment is stabilized by both Ag and Na doping and this can be explained by considering the changes in band filling due to substitution as suggested by density functional theory (DFT) calculations. We thus show that chemical substitution and the subsequent changes in band filling could be a pathway to tune the magnetic ground state and to stabilize a ferromagnetic phase in EuCd$_2$As$_2$.

cond-mat.mtrl-sci↗

arXiv2022

TSAA: A Two-Stage Anchor Assignment Method towards Anchor Drift in Crowded Object Detection

Among current anchor-based detectors, a positive anchor box will be intuitively assigned to the object that overlaps it the most. The assigned label to each anchor will directly determine the optimization direction of the corresponding prediction box, including the direction of box regression and category prediction. In our practice of crowded object detection, however, the results show that a positive anchor does not always regress toward the object that overlaps it the most when multiple objects overlap. We name it anchor drift. The anchor drift reflects that the anchor-object matching relation, which is determined by the degree of overlap between anchors and objects, is not always optimal. Conflicts between the fixed matching relation and learned experience in the past training process may cause ambiguous predictions and thus raise the false-positive rate. In this paper, a simple but efficient adaptive two-stage anchor assignment (TSAA) method is proposed. It utilizes the final prediction boxes rather than the fixed anchors to calculate the overlap degree with objects to determine which object to regress for each anchor. The participation of the prediction box makes the anchor-object assignment mechanism adaptive. Extensive experiments are conducted on three classic detectors RetinaNet, Faster-RCNN and YOLOv3 on CrowdHuman and COCO to evaluate the effectiveness of TSAA. The results show that TSAA can significantly improve the detectors' performance without additional computational costs or network structure changes.

cs.CV↗

arXiv2022

Thickness and twist angle dependent interlayer excitons in metal monochalcogenide heterostructures

Interlayer excitons, or bound electron-hole pairs whose constituent quasiparticles are located in distinct stacked semiconducting layers, are being intensively studied in heterobilayers of two dimensional semiconductors. They owe their existence to an intrinsic type-II band alignment between both layers that convert these into p-n junctions. Here, we unveil a pronounced interlayer exciton (IX) in heterobilayers of metal monochalcogenides, namely gamma-InSe on epsilon-GaSe, whose pronounced emission is adjustable just by varying their thicknesses given their number of layers dependent direct bandgaps. Time-dependent photoluminescense spectroscopy unveils considerably longer interlayer exciton lifetimes with respect to intralayer ones, thus confirming their nature. The linear Stark effect yields a bound electron-hole pair whose separation d is just (3.6 \pm 0.1) Å with d being very close to dSe = 3.4 Å which is the calculated interfacial Se separation. The envelope of IX is twist angle dependent and describable by superimposed emissions that are nearly equally spaced in energy, as if quantized due to localization induced by the small moiré periodicity. These heterostacks are characterized by extremely flat interfacial valence bands making them prime candidates for the observation of magnetism or other correlated electronic phases upon carrier doping.

cond-mat.mes-hall↗

arXiv2022

Pressure-temperature phase diagram of CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ for $x$=0.024

Resistance measurements on single crystals of CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ ($x$ = 0.024) were performed under hydrostatic pressure up to 5.15 GPa. The pressure dependence of the magnetic and superconducting transition temperatures and that of the superconducting upper critical field are reported. Our results show that upon increasing pressure, the magnetic transition temperature $T_{N}$ is suppressed, whereas the superconducting transition temperature $T_{c}$ first increases and then decreases, exhibiting a maximum at a pressure $p_c$ corresponding to the intersection of the $T_{N}$($p$) and $T_{c}$($p$) lines. In addition, a minimum in the normalized slope of the superconducting upper critical field as well as a change in the pressure dependence of the inferred superconducting coherence length are observed at $p_c$, suggesting a difference in the Fermi surface of the paramagnetic and antiferromagnetic states. Finally, CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ ($x$ = 0.024) likely goes through a half-collapsed-tetragonal phase transition at $\sim$ 4.3 GPa, further demonstrating that the half-collapsed-tetragonal transition pressure in the CaKFe$_4$As$_4$ system is relatively insensitive to transition metal substitution.

cond-mat.supr-con↗

arXiv2022

Toward Clinically Assisted Colorectal Polyp Recognition via Structured Cross-modal Representation Consistency

The colorectal polyps classification is a critical clinical examination. To improve the classification accuracy, most computer-aided diagnosis algorithms recognize colorectal polyps by adopting Narrow-Band Imaging (NBI). However, the NBI usually suffers from missing utilization in real clinic scenarios since the acquisition of this specific image requires manual switching of the light mode when polyps have been detected by using White-Light (WL) images. To avoid the above situation, we propose a novel method to directly achieve accurate white-light colonoscopy image classification by conducting structured cross-modal representation consistency. In practice, a pair of multi-modal images, i.e. NBI and WL, are fed into a shared Transformer to extract hierarchical feature representations. Then a novel designed Spatial Attention Module (SAM) is adopted to calculate the similarities between the class token and patch tokens %from multi-levels for a specific modality image. By aligning the class tokens and spatial attention maps of paired NBI and WL images at different levels, the Transformer achieves the ability to keep both global and local representation consistency for the above two modalities. Extensive experimental results illustrate the proposed method outperforms the recent studies with a margin, realizing multi-modal prediction with a single Transformer while greatly improving the classification accuracy when only with WL images.

cs.CV↗

arXiv2021

Magnetization and magneto-transport measurements on CeBi single crystals

We report the synthesis of CeBi single crystals out of Bi self flux and a systematic study of the magnetic and transport properties with varying temperature and applied magnetic fields. From these $R(T,H)$ and $M(T,H)$ data we could assemble the field-temperature ($H-T$) phase diagram for CeBi and visualize the three dimensional $M-T-H$ surface. In the phase diagram, we identify regions with well defined magnetization values, and identify a new phase region. The magnetoresistance (MR) in the low temperature regime shows, above $6~$T a power-law, non-saturated behavior with large MR ($\sim 3\times10^5 \%$ at $2~$K and $13.95~$T), along with Shubnikov-de Haas oscillations. With increasing temperatures, MR decreases, and then becomes negative for $T\gtrsim 10~$K. This crossover in MR seems to be unrelated to any specific magnetic or metamagnetic transitions, but rather is associated with changing from a low-temperature normal metal regime with little or no scattering from the Ce$^{3+}$ moments and an anomalously large MR, to an increased scattering from local Ce moments and a negative MR as temperature increases.

cond-mat.str-el↗

arXiv2021

Small moment antiferromagnetic ordering in single crystalline La2Ni7

Single crystals of La2Ni7 have been grown out of a binary, La-Ni melt. Temperature dependent, zero magnetic field, specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T1 = 61.0 \pm 0.2 K, T2 = 56.5 \pm 0.2 K and T3 = 42.2 \pm 0.2 K. The three specific heat anomalies found at these temperatures qualitatively have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μB/Ni; both observations strongly suggesting the AFM order is associated with very small, itinerant, moments. Anisotropic, H||c and H{\perp}c, ρ(H) and M(H) isotherms as well as constant field, ρ(T) and M(T) sweeps manifest signatures of multiple phase lines and result in H-T phase diagrams that are clearly anisotropic. Analysis of M(T) and M(H) data allow for the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the crystallographic c-axis, and the higher temperature, T2 < T < T1, state as having a finite ferromagnetic component. In addition, the metamagnetic transition at low temperatures, for H applied along the crystallographic c-axis (H||c) appears to be a near classic example of a spin-flop transition, resulting in a field stabilized antiferromagnetic state with the moments ordered perpendicular to the c-axis. Although the small moment ordering, and existence of multiple phase transitions in field and temperature, suggesting an energetic proximity of these states, could foretell a degree of pressure sensitivity, our measurements of R(T) for applied pressures up to 2.0 GPa indicate that there is very little pressure dependence of T1, T2 and T3.

cond-mat.str-el↗

arXiv2021

Colorectal Polyp Classification from White-light Colonoscopy Images via Domain Alignment

Differentiation of colorectal polyps is an important clinical examination. A computer-aided diagnosis system is required to assist accurate diagnosis from colonoscopy images. Most previous studies at-tempt to develop models for polyp differentiation using Narrow-Band Imaging (NBI) or other enhanced images. However, the wide range of these models' applications for clinical work has been limited by the lagging of imaging techniques. Thus, we propose a novel framework based on a teacher-student architecture for the accurate colorectal polyp classification (CPC) through directly using white-light (WL) colonoscopy images in the examination. In practice, during training, the auxiliary NBI images are utilized to train a teacher network and guide the student network to acquire richer feature representation from WL images. The feature transfer is realized by domain alignment and contrastive learning. Eventually the final student network has the ability to extract aligned features from only WL images to facilitate the CPC task. Besides, we release the first public-available paired CPC dataset containing WL-NBI pairs for the alignment training. Quantitative and qualitative evaluation indicates that the proposed method outperforms the previous methods in CPC, improving the accuracy by 5.6%with very fast speed.

cs.CV↗

arXiv2021

Pressure-induced ferromagnetism in the topological semimetal EuCd$_2$As$_2$

The antiferromagnet and semimetal EuCd$_2$As$_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$-axis in EuCd$_2$As$_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd$_2$As$_2$. For that, we present specific heat, transport and $μ$SR measurements under hydrostatic pressure up to $\sim\,2.5\,$GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd$_2$As$_2$ changes from in-plane antiferromagnetic (AFM$_{ab}$) to ferromagnetic at a critical pressure of $\,\approx\,$2\,GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane (FM$_{ab}$). The AFM$_{ab}$-FM$_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd$_2$As$_2$ moves closer to the sought-for FM$_c$ state (moments $\parallel$ $c$) with increasing pressure further. We predict that a pressure of $\approx$\,23\,GPa will stabilize the FM$_c$ state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd$_2$As$_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

cond-mat.str-el↗

arXiv2020

Formation of short-range magnetic order and avoided ferromagnetic quantum criticality in pressurized LaCrGe$_3$

LaCrGe$_3$ has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. By combining thermodynamic, transport, x-ray and neutron scattering as well as $μ$SR measurements, we refined the temperature-pressure phase diagram of LaCrGe$_3$. We provide thermodynamic evidence (i) for the first-order character of the FM transition when it is suppressed to low temperatures and (ii) for the formation of new phases at high pressures. From our microscopic data, we infer that short-range FM ordered clusters exist in these high-pressure phases. These results suggest that LaCrGe$_3$ is a rare example, which fills the gap between the two extreme limits of avoided FM quantum criticality in clean and strongly disordered metals.

cond-mat.str-el↗

arXiv2020

Avoided ferromagnetic quantum critical point in pressurized La$_5$Co$_2$Ge$_3$

We present the pressure-temperature phase diagram La$_5$Co$_2$Ge$_3$ up to $\sim$ 5\,GPa, which was constructed from magnetization, resistivity and specific heat measurements. At ambient pressure, La$_5$Co$_2$Ge$_3$ is an itinerant ferromagnet with a Curie temperature $T_\textrm C\sim$ 4\,K. Upon increasing pressure up to $\sim$ 1.7\,GPa, $T_\textrm C$ is suppressed down to $\sim$ 3\,K. Upon further increasing pressure, our results suggest that La$_5$Co$_2$Ge$_3$ enters a different low-temperature ground state. The corresponding transition temperature, $T^*$, has a nonmonotonic pressure dependence up to $\sim$ 5\,GPa. Our results demonstrate that the ferromagnetic quantum critical point in La$_5$Co$_2$Ge$_3$ is avoided by the appearance of a different, likely magnetically ordered state that has an antiferromagnetic component.

cond-mat.str-el↗

arXiv2020

Characterization of the pressure coefficient of manganin and temperature evolution of pressure in piston-cylinder cells

We report measurements of the temperature- and pressure-dependent resistance, $R(T,p)$, of a manganin manometer in a $^4$He-gas pressure setup from room temperature down to the solidification temperature of $^4$He ($T_\textrm {solid}\sim$ 50 K at 0.8 GPa) for pressures, $p$, between 0 GPa and $\sim$0.8 GPa. The same manganin wire manometer was also measured in a piston-cylinder cell from 300 K down to 1.8 K and for pressures between 0 GPa to $\sim$2 GPa. From these data, we infer the temperature and pressure dependence of the pressure coefficient of manganin, $α(T,p)$, defined by the equation $R_p = (1+αp) R_0$ where $R_0$ and $R_p$ are the resistance of manganin at ambient pressure and finite pressure, respectively. Our results indicate that upon cooling $α$ first decreases, then goes through a broad minimum at $\sim$120 K and increases again towards lower temperatures. In addition, we find that $α$ is almost pressure-independent for $T\gtrsim$60 K up to $p\sim$2 GPa, but shows a pronounced $p$ dependence for $T\lesssim$60K. Using this manganin manometer, we demonstrate that $p$ overall decreases with decreasing temperature in the piston-cylinder cell for the full pressure range and that the size of the pressure difference between room temperature and low temperatures ($T=1.8$ K), $Δp$, decreases with increasing pressure. We also compare the pressure values inferred from the magnanin manometer with the low-temperature pressure, determined from the superconducting transition temperature of elemental lead (Pb). As a result of these data and analysis we propose a practical algorithm to infer the evolution of pressure with temperature in a piston-cylinder cell.

cond-mat.mtrl-sci↗

arXiv2020

Tuning of charge density wave transitions in LaAu$_x$Sb$_2$ by pressure and Au-stoichiometry

Two charge density wave transition can be detected in LaAu$_x$Sb$_2$ at ~ 110 and ~ 90 K by careful electrical transport measurements. Whereas control of the Au site occupancy in LaAu$_x$Sb$_2$ (for 0.9 < x < 1.0) can suppress each of these transitions by ~ 80 K, the application of hydrostatic pressure can completely suppress the lower transition by ~ 10 kbar and the upper transition by ~ 17 kbar. Clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero.

cond-mat.mtrl-sci↗

arXiv2020

Hydrostatic and uniaxial pressure tuning of iron-based superconductors: Insights into superconductivity, magnetism, nematicity and collapsed tetragonal transitions

Iron-based superconductors are well-known for their intriguing phase diagrams, which manifest a complex interplay of electronic, magnetic and structural degrees of freedom. Among the phase transitions observed are superconducting, magnetic, and several types of structural transitions, including a tetragonal-to-orthorhombic and a collapsed-tetragonal transition. In particular, the widely-observed tetragonal-to-orthorhombic transition is believed to be a result of an electronic order that is coupled to the crystalline lattice and is, thus, referred to as nematic transition. Nematicity is therefore a prominent feature of these materials, which signals the importance of the coupling of electronic and lattice properties. Correspondingly, these systems are particularly susceptible to tuning via pressure (hydrostatic, uniaxial, or some combination). We review efforts to probe the phase diagrams of pressure-tuned iron-based superconductors, with a strong focus on our own recent insights into the phase diagrams of several members of this material class under hydrostatic pressure. These studies on FeSe, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ were, to a significant extent, made possible by advances of what measurements can be adapted to the use under differing pressure environments. We point out the potential impact of these tools for the study of the wider class of strongly correlated electron systems.

cond-mat.supr-con↗

arXiv2020

Pressure tuning of structural and magnetic transitions in EuAg$_4$As$_2$

We report temperature dependent measurements of ambient pressure specific heat, magnetic susceptibility, anisotropic resistivity and thermal expansion as well as in-plane resistivity under pressure up to 20.8 kbar on single crystals of EuAg$_4$As$_2$. Based on thermal expansion and in-plane electrical transport measurements at ambient pressure this compound has two, first order, structural transitions in 80 - 120 K temperature range. Ambient pressure specific heat, magnetization and thermal expansion measurements show a cascade of up to seven transitions between 8 and 16 K associated with the ordering of the Eu$^{2+}$ moments. In-plane electrical transport is able to detect more prominent of these transitions: at 15.5, 9.9, and 8.7 K as well as a weak feature at 11.8 K at ambient pressure. Pressure dependent electrical transport data show that the magnetic transitions shift to higher temperatures under pressure, as does the upper structural transition, whereas the lower structural transition is suppressed and ultimately vanishes. A jump in resistivity, associated with the upper structural transition, decreases under pressure with an extrapolated disappearance (or a change of sign) by 30-35 kbar. In the 10 - 15 kbar range a kink in the pressure dependence of the upper structural transition temperature as well as the high and low temperature in-plane resistivities suggest that a change in the electronic structure may occur in this pressure range. The results are compared with the literature data for SrAg$_4$As$_2$.

cond-mat.mtrl-sci↗

arXiv2019

Measurements of elastoresistance under pressure by combining in-situ tunable quasi-uniaxial stress with hydrostatic pressure

Uniaxial stress, as well as hydrostatic pressure are often used to tune material properties in condensed matter physics. Here, we present a setup which allows for the study of the combined effects of quasi-uniaxial stress and hydrostatic pressure. Following earlier designs for measurements under finite stress at ambient pressure (e.g., Chu et al., Science 337, 710 (2012)), the present setup utilizes a piezoelectric actuator to change stress in situ inside the piston-cylinder pressure cell. We show that the actuator can be operated over the full temperature (from 30K up to 260 K) and pressure range (up to ~2GPa), resulting in a clear and measurable quasi-uniaxial strain. To demonstrate functionality, measurements of the elastoresistance (i.e., the change of resistance of a sample as a response to quasi-uniaxial strain) under finite hydrostatic pressure on the iron-based compound BaFe$_2$As$_2$ are presented as a proof-of-principle example, and discussed in the framework of electronic nematicity. Overall, this work introduces the combination of in situ tunable quasi-uniaxial stress and large (up to ~2GPa) hydrostatic pressure as a powerful combination in the study of novel electronic phases. In addition, it also points towards further technical advancements which can be made in the future.

cond-mat.supr-con↗

arXiv2019

Role of Fermi surface for the pressure-tuned nematic transition in the BaFe$_2$As$_2$ family

The tetragonal-to-orthorhombic phase transition at $T_s$, which precedes the antiferromagnetic phase transition at $T_N$ in many iron-based superconductors, is considered as one of the manifestations of electronic \textit{nematic} order. By constructing temperature-pressure phase diagrams of pure and Co-doped BaFe$_2$As$_2$, we study the relation of $T_s$ and $T_N$ under pressure $p$. Our data disclose two qualitatively different regimes in which $ΔT\,=\,T_s-T_N$ either increases or decreases with $p$. We provide experimental evidence that the transition between the two regimes may be associated with sudden changes of the Fermi surface topology. Therefore, our results not only support the electronic origin of the structural order, but also emphasize the importance of details of the Fermi surface for the evolution of nematic order under pressure.

cond-mat.supr-con↗

arXiv2019

Understanding the Disharmony between Weight Normalization Family and Weight Decay: $ε-$shifted $L_2$ Regularizer

The merits of fast convergence and potentially better performance of the weight normalization family have drawn increasing attention in recent years. These methods use standardization or normalization that changes the weight $\boldsymbol{W}$ to $\boldsymbol{W}'$, which makes $\boldsymbol{W}'$ independent to the magnitude of $\boldsymbol{W}$. Surprisingly, $\boldsymbol{W}$ must be decayed during gradient descent, otherwise we will observe a severe under-fitting problem, which is very counter-intuitive since weight decay is widely known to prevent deep networks from over-fitting. In this paper, we \emph{theoretically} prove that the weight decay term $\frac{1}{2}λ||{\boldsymbol{W}}||^2$ merely modulates the effective learning rate for improving objective optimization, and has no influence on generalization when the weight normalization family is compositely employed. Furthermore, we also expose several critical problems when introducing weight decay term to weight normalization family, including the missing of global minimum and training instability. To address these problems, we propose an $ε-$shifted $L_2$ regularizer, which shifts the $L_2$ objective by a positive constant $ε$. Such a simple operation can theoretically guarantee the existence of global minimum, while preventing the network weights from being too small and thus avoiding gradient float overflow. It significantly improves the training stability and can achieve slightly better performance in our practice. The effectiveness of $ε-$shifted $L_2$ regularizer is comprehensively validated on the ImageNet, CIFAR-100, and COCO datasets. Our codes and pretrained models will be released in https://github.com/implus/PytorchInsight.

cs.LG↗